1. Field of the Invention
The present invention relates to a plasma treatment apparatus for treating an object with plasma made up of plural streamers, and plasma treatment method using the plasma, which can be used in various application fields of performing surface-cleaning or surface-modification to electronic parts and semiconductor parts, cleaning bonding portions of CSP (Chip Size Package) and BGA (Ball Grid Array) to improve bonding strength and adhesion with sealing materials, cleaning liquid-crystal panels, improving bonding properties of ACF (Anisotropic Conductive Film) with the liquid-crystal panels, improving adhesion and printing properties of film-like substrates, resist ashing, reducing and removing metal oxides on electrodes to improve bonding properties.
2. Disclosure of the Prior Art
In the past, atmospheric pressure plasma has been used to perform a surface treatment on an object. For example, surface treatments for treating an object with a plasma jet generated under atmospheric pressure are disclosed in Japanese Patent Early Publications [KOKAI] Nos. 3-219082, 4-212253, 4-334543, and 6-108257. According to these surface treatments, it is possible to accurately treat only a required region on the object with the plasma jet.
In addition, methods of efficiently treating a large-area substrate with a wide plasma jet are disclosed in Japanese Patent Early Publications [KOKAI] Nos. 4-358076 and 9-232293. In the former, a pair of electrodes having a dielectric layer are arranged in a face-to-face relation such that the dielectric layer is put between the adjacent electrodes, and glow discharge of a rare gas is stably generated under atmospheric pressure by applying a high-frequency electric power between the electrodes. A resultant liner plasma jet is injected on the object. In the latter, a pair of electrodes are arranged around the perimeter of a flat discharge tube, and a liner plasma jet is injected from one end of the discharge tube. However, when the plasma jet is spot-injected on an object such as BGA substrates (Ball Grid Array) or films having a relatively large surface area to be treated, it is needed to allow the plasma jet to scan throughout the surface of the object. In such a case, there is a problem of lowering the treatment efficiency. On the other hand, when using the linear plasma of the stable glow discharge for the surface treatment, the treatment efficiency can be improved to some extent. However, since a current density of the glow discharge plasma is relatively low, which means that a density of active species such as radicals and ions in the plasma is low, the treatment speed is not still enough to treat the large-area object.
By the way, it is generally said that glow discharge is a spatially uniform discharge over time, which has a relatively small current density and low emission brightness. As a different discharge from the glow discharge, there are streamers, each of which is an unstable filament-like discharge having a relatively larger current density and higher emission brightness than the glow discharge. Since the high current density of the streamer presents a high density of the active species, the streamers have a potential of increasing the treatment speed by a large amount.
However, the unstable discharge state of the streamers easily causes considerable variations in treatment effects. That is, the treatment speed is much higher at downstream of a generation site of the individual streamer than at downstream of a non-generation site thereof in the discharge space. The variations in treatment effects on the object caused by this difference in the treatment speed are a principal reason for preventing the application of the streamers to surface treatment technology.
In addition, when active species in plasma discharge generated in a plasma-generation chamber are released into the atmosphere, they recombine with nitrogen and oxygen, and most of them become inactive. In this case, as the time required for a plasma jet including the active species to reach a surface to be treated of an object increases, and the contact area between the plasma jet and atmospheric gases increases, the active species easily become inactive, and the treatment effects brought on the object by the plasma jet lower. Therefore, when the plasma jet has a laterally rectangular cross section, variations in treatment effects easily occur because the contact area between laterally opposite ends of the plasma jet and the atmospheric gases is larger than the contact area between the center thereof and the atmospheric gases. Moreover, when the object has projections or depressions on the surface, the time required for the plasma jet to reach the depressions is longer than the time required for the plasma jet to reach the projections, so that variations in treatment effects on the object further increase. This increase in the variations results in the deterioration of the performance of the object, e.g., a decrease in bonding reliability. Thus, it is desired to improve the uniformity of the plasma treatment.
In view of the above facts, the present invention has achieved success in practical use of a plasma surface treatment by serving the purpose of improving distribution uniformity of the plural streamers in the discharge space and uniformly treating an object with plasma made up of the plural streamers at a high treatment speed from the viewpoints of apparatus and method. Therefore, a first object of the present invention is to provide the plasma treatment apparatus for uniformly treating the object with the plasma made up of the plural streamers, which comprises:
a tubular vessel made of an electrically-insulating material and having a laterally elongated cross section, which has a gas inlet at its one end and a plasma outlet at the other end;
a pair of electrodes, which are arranged such that electric flux lines develop substantially in an axial direction of the tubular vessel when one of an AC voltage and a pulse voltage is applied between the electrodes;
a gas supply for supplying a streamer generation gas into the tubular vessel though the gas inlet;
a power source for applying one of the AC voltage and the pulse voltage between the electrodes to generate the plural streamers of the gas in the tubular vessel; and
a plasma uniform means for making the plural streamers uniform in a lateral direction of the laterally elongated cross section of the tubular vessel to provide the plasma from the plasma outlet.
It is preferred that the plasma uniform means is provided by (1) the tubular vessel formed such that an aperture area of the gas inlet is smaller than a cross-sectional area immediately below the gas inlet of the tubular vessel; (2) a turbulence generating member disposed in the tubular vessel, which has a shape capable of generating a turbulence of the streamer generation gas in the tubular vessel; or (3) the tubular vessel formed such that the laterally elongated cross section of the tubular vessel has an inner width of 1 to 5 mm.
A second object of the present invention is to provide a plasma treatment method using the plasma made up of the plural streamers and having the capability of serving the above purpose. That is, the surface treatment process comprises the following steps of:
supplying a streamer generation gas into a tubular vessel made of an electrically-insulating material and having a laterally elongated cross section, which has a gas inlet at its one end and a plasma outlet at the other end;
generating plural streamers under atmospheric pressure or a pressure near the atmospheric pressure in the tubular vessel by applying one of an AC voltage and a pulse voltage between a pair of electrodes arranged such that electric flux lines develop substantially in an axial direction of the tubular vessel;
distributing the plural streamers uniformly in a lateral direction of the laterally elongated cross section of the tubular vessel to obtain plasma in the tubular vessel; and treating an object with the plasma provided from the plasma outlet of the tubular vessel.
It is preferred that the step of making the plural streamers uniform is performed by (1) supplying the streamer generation gas into the tubular vessel such that a flow of the streamer generation gas in the tubular vessel has a Reynolds number of 1700 or more under non-discharge condition, or (2) supplying the streamer generation gas into the tubular vessel such that a flow velocity of the streamer generation gas in the axial direction of the tubular vessel is 5 m/sec or more.
A further object of the present invention is to provide a plasma treatment apparatus having a treatment-effect uniform means for uniformly carrying out a plasma treatment on the entire surface region to be treated of the object by controlling time required for a plasma jet to reach the surface region, in order to obtain uniform treatment effects on the object. That is, the plasma treatment apparatus comprises a tubular vessel made of an electrically-insulating material and having a gas inlet at its one end and a plasma outlet at the other end, a pair of electrodes, a gas supply for supplying a plasma generation gas into the tubular vessel though the gas inlet, a power source for applying one of an AC voltage and a pulse voltage between the electrodes to generate the plasma of the gas in the tubular vessel, and the treatment-effect uniform means for facilitating the uniformity of plasma-treatment effects on the object.
These and still other objects and advantages will become apparent from the following detail description of preferred embodiments and Examples of the invention, referring to the attached drawings.